The communication between subscribers, such as ISDN/PSTN subscribers for example, conducted hitherto by way of conventional circuit-switched telecommunication networks is conducted by way of IP networks in the case of later communication architectures. Conventional ISDN/PSTN terminal devices connected to terminating devices (Integrated Access Devices, IAD) for xDSL paths or else also IP based terminal devices employing corresponding IP based signaling (H.323/SIP), for example, can be used as terminal devices.
The aforementioned terminating devices are controlled by control devices or control units, which can be part of a packet-oriented switching system. A large number of connection-mode IP communication connections (TCP, SCTP for example) are routed between control unit and terminating device (25,000 for example). In order to control the connections, connection-related data (such as Transmission Control Block, source addresses, destination addresses, the status of the connection or statistical data etc., for example) must be stored in the terminating device and the control unit. Non-availability situations affecting the control unit (resulting from a failure (fault), Reset command or manual reconfiguration, for example) are detected in the terminating device. In the case of connection-mode IP connections this now presents the problem whereby these connections can only then continue to be used when the control unit becomes available again if all the connection-related data for the connections is still present in the control unit as previously. However, this is not generally the case when a control unit becomes available again.
When the control unit fails, the connection-related data is also lost. This means that a control unit which becomes available again that is receiving messages by way of the still active—from the viewpoint of the terminating device—connections is unable to assign these to any IP connection. As a result of the non-availability of the control unit, from the latter's viewpoint the communication connection with the terminating device becomes inactive. The messages received by the latter become out-of-the-blue messages.
With regard to the prior art, the IP connection must then be reestablished or recovered (Restart). However, this also means that all incoming messages from the time of the failure up to the recovery of the IP connection will be lost because they are discarded by the transport layer. The establishment or recovery of an individual IP connection takes place very quickly; the establishment of a plurality of IP connections takes place sequentially. Since the communication by way of the control device takes place over a large number of IP connections, a significant period of time is therefore required overall depending on their number in order to establish all the IP connections. At least the last terminating device thus remains faulty until such time as all IP connections have been reestablished.